1. Field of the Invention
The present invention relates to video recording, and more particularly refers to an apparatus which may be utilized to synchronize two or more color video cameras or other video devices for simultaneous operation.
2. Prior Art
Because of their many advantages, video cameras are used almost exclusively for photographing programs which are to be utilized for television broadcasting, as well as for video recording.
In a television studio, many color video signals are brought together at one point where they are selected, mixed, and/or altered. This point may be called the "switcher". The resultant signal leaves the switcher and goes to a transmitter for "airing" or to a video tape recorder for storage. In order for the switcher to do an adequate and acceptable job, it is required that all video signals at the switcher input be synchronized.
The human eye decodes light as parallel data decoding hue, saturation, and luminance. In contrast, a television system scans the scene and encodes this data as serial information. In order for a television system to convert parallel information to serial, a top (vertical sync) and a left (horizontal sync) must be determined. Additionally, chrominance information (hue and saturation) must be encoded and referenced to color sync. These three sync units are required so that the television receiver can decode the encoded information so that it can faithfully reproduce the scene as the camera views it.
One conventional method of synchronizing signals from several cameras is to have a master sync generator, usually close to the switcher. The master sync generator ator signals are distributed to all of the equipment which originates the video feeding the switcher, i.e., television cameras, title generators, film chains, etc. These cameras, etc., which are synchronized to each other are referred to as being "driven". By being driven, the video equipment is forced to stay in step with other driven video equipment in the system. As it takes only a small amount of time for a signal to go through a cable, and since this time is proportional to the length of the cable, all cables from synchronized, composite video must be electrically the same length from their source to the switcher. Normal practice dictates that the longest video line is run directly into the switcher, the other video lines are then electrically lengthened in order to match. This normally is a tedious operation. In contrast, in the present invention, it is accomplished by automatic timing.
A relatively inexpensive camera relies on digital techniques to generate vertical, horizontal, and color sync within the source. One of these cameras combined with a video tape recorder can provide a consumer with a means of making video home movies. As these cameras have no simple means of being externally synchronized, more than one video source cannot normally be used.
Presently, many different versions of N.T.S.C. (National Television Systems Committee) internal sync generators are in common use. However, most are of the digital type and derive the various synchronizing components from a 14.318180 MHz crystal-controlled oscillator, and use a countdown formula to arrive at the necessary color sync and drive components. Some equipment, such as inexpensive color cameras, use only a single, specially-designed, integrated circuit for a sync, with the only input being the 14.3 MHz crystal input.
The apparatus used for synchronizing video cameras in other forms of the prior art also have a number of disadvantages. The electronic equipment must be custom tailored to fit each make and model camera. This requires an inordinant amount of time to be spent in final engineering of the system. The custom tailored equipment is much too expensive and complicated. It depends on a non-standard, two-conductor, shielded drive cable between the camera and the synchronizer. The camera modification requires the incorporation of a switching-type jack to allow the camera to revert to normal, free running operation when disconnected from the synchronizing unit. The use of such a jack in itself causes reliability problems. The camera modification generally contains a negative voltage power supply which at times causes interference problems. Additionally, the cameras must be matched to the synchronizer with which they are to be used. There is no uniformity between cameras. A further disadvantage is the lock-up (synchronization) time is generally too long, often five minutes or more. This results from the fact that the apparatus simply speeds up or slows down the driven camera voltage-controlled oscillator. Further, the vertical odd/even fields detector circuits are unstable and require frequent realignment. After the unit locks-up (synchronizes the cameras), it sometimes unlocks and starts hunting again when the cameras are moved about (changing scenes and light levels). Further, after even a slight adjustment, one often must wait a long time for the unit to re-lock, to determine whether or not it is working properly. A further disadvantage of the prior art is that the voltage-controlled oscillator which drives voltage to the driven camera is often very noisy and must be carefully adjusted. Another disadvantage is that because of the large number of component parts which must be utilized, the circuit board is too large, crowded and complicated, making assembly and final testing extremely difficult. Final alignment requires much expertise and can only be accomplished by an engineer with very sophisticated equipment. Additionally, the power supply operates at maximum capacity, leaving no reserve for expansion to add additional driven camera outputs. Finally, the inherent color phase correction circuits are sometimes unstable and repeatability of system set-up can be a problem. Additionally, the manual phase control operation may be erratic.
As the term video camera is used herein, it is intended that the term also include other video devices.